Flying After Diving -- Cracking the DCS Code

Hyperbaric researchers shed new light on the risks of flying after diving

By Anthony K. Almon

It's been a wonderful week - probably the best dive vacation you've ever taken. The fish have been plentiful, and the reefs are teeming with activity. There's a dive this afternoon at 5 p.m., and you really wanted to have one last look at that enormous school of jacks out on the reef. There's just one minor problem. Your flight back home is at 7 a.m. tomorrow morning, and you don't want to suffer decompression sickness (DCS) because you flew too soon after diving...

Should you go on the dive? The U.S. Navy tables recommend that you wait at least two hours before you board a plane after diving; the U.S. Air Force says you should wait 24 hours; DAN recommends a 12-hour minimum surface interval before flying. Which guideline should you follow? If you fly after diving, what you really want to know is:

What are my chances (or probability) of experiencing a decompression injury; and

If I do get hit, how severe might this injury be?

These two factors, the probability and the severity of injury, help determine the risk you are willing to take. So your next question might be: How much risk is right for you? You need to consider many factors before making your decision. The number of previous dives you've made on this dive trip, your general health and your age are but a few points to ponder. What if you were returning to a location like Raleigh-Durham, N.C., where a chamber is available? Would you be willing to take more of a chance? The risk that some divers are willing to accept may be absolutely unacceptable to you.

The information you act upon is very important, so it's worth looking into. Unfortunately, in many cases little or no information is available, and we act on nothing more than speculation and hearsay. What do we really know about the flying after diving question? Because so little data is available to support the current guidelines, DAN is conducting an experimental research study at the F.G. Hall Hypo/Hyperbaric Center of Duke Medical Center to help answer this question. Although the study is far from complete, we will use the preliminary results here to discuss safety and the practical application of the data.

What we want to know is how the probability of DCS changes as we increase the length of the preflight surface interval. Our first series of experiments consisted of one dive followed by a flight. A dive to 60 feet sea water (fsw)/18 meters for 55 minutes was performed, followed by a predetermined surface interval. The altitude exposure of the flight was 8,000 feet/2,440 meters for four hours. Eight thousand feet is the maximum cabin altitude allowed in a commercial aircraft.

To produce meaningful information about DCS and the preflight surface interval, some DCS must occur. This information is used to estimate the probability of decompression sickness. Note that we cannot actually measure probability - we can only estimate it statistically from experimental data. Decompression sickness that occurs in laboratory experiments is generally mild and easily treated, but we have a responsibility not to expose our volunteer research subjects - who are recreational divers - to experiments that are likely to produce DCS that does not resolve completely. The only way we could completely eliminate risk is not to conduct the experiments at all, but this leaves us where we started - with uncertain information about flying after diving. And without research, thousands of people would continue to experiment on themselves by flying home from vacation without the benefit of medical supervision provided in a laboratory study.

The protection of volunteer divers is a very important issue. We subject people to experimental protocols that we know will produce some DCS, so what do we do to protect these volunteers? First, before anyone can participate in one of our studies, a comprehensive medical history and physical are performed by a hyperbaric physician. Anyone with severe medical problems is not allowed to participate. Some of the problems which may disqualify a diver are neurological problems, chronic injuries and lung conditions that may cause gas trapping. Additionally, because diving is a hazard to an unborn child, pregnant divers are not permitted, and all women of childbearing potential must undergo a pregnancy test.

The next issue is to decide when DCS has occurred. Since there is no lab test for DCS, we must diagnose it by the presence of signs and symptoms. Unfortunately, the milder forms of DCS may have signs and symptoms that are the same as some daily-life aches and pains from which we all occasionally suffer - particularly as we get older. Because of this uncertainty, we ask divers to report any and all symptoms, no matter how mild or seemingly unrelated to the experiment. After the experiment, these are broken into three categories: "Not DCS," "Ambiguous DCS" and "Definite DCS."

"Not DCS" refers to signs or symptoms that are clearly unrelated to the experiment, such as the diver who sprained his ankle playing basketball during the preflight surface interval. (We no longer allow sports during the surface intervals.) "Ambiguous DCS" refers to signs and symptoms that may have lasted only a short time, were very mild and uncertain in the judgment of the hyperbaric physician, or may not have responded to recompression therapy. "Definite DCS" refers to clear and certain signs and symptoms that improve or resolve completely with recompression.

Within the category of Definite DCS there are two types: pain-only and neurological. We generally worry more about neurological symptoms than joint pain. Neurological signs and symptoms that have occurred in our studies have included numbness, tingling, weakness, confusion and visual disturbances, all of which resolved with recompression. Obviously, we want a lower probability of neurological DCS than either joint pain or Ambiguous DCS.

One of the most important factors in providing diver protection is the diver himself. Complete and timely reporting of any and all signs and symptoms ensures the earliest possible recompression, if needed, and the collection of good-quality data. To reduce the chances of testing a preflight surface interval that may have too high a DCS probability, we establish acceptance and rejection rules that define how many times we should test a surface interval based on the occurrence of Definite DCS in previous tests. These rules must be approved by the Duke Medical Center Institutional Review Board, which oversees all human experimentation.

The acceptance/rejection rules for a surface interval in the Flying After Diving study, based on Definite DCS, are:

Accept

0 pain-only incidents in 23 trials

1 pain-only incidents in 25 trials

2 pain-only incidents in 46 trials

Reject

Any neurological incident

2 pain-only incidents in 10 trials

3 pain-only incidents

Note that acceptance of a surface interval applies only within the Flying After Diving study and does not imply acceptance for actual use in recreational diving. The three-hour surface interval had one pain-only and two neurological incidents of Definite DCS, and seven incidents of Ambiguous DCS. A single incident of neurological Definite DCS occurred at six and at nine hours, and one Ambiguous DCS incident occurred at six hours. Because of neurological DCS, the surface intervals for three, six and nine hours were rejected. Ten-, 11- and 12-hour surface intervals, however, were considered acceptable according to the acceptance/rejection rules described above. The 10-hour surface interval produced one ambiguous symptom in 23 trials, but the 11- and 12-hour surface intervals had no symptoms in 23 and 27 exposures.

As with all research, proof lies in numbers. The more trials made at a given surface interval increases the confidence in the accuracy of the data. For example, if you performed 100 trials, you would be more confident in the accuracy of your results than if you performed only 10 trials.

There were three Definite DCS-incidents in 36 trials of the three-hour surface interval, for an 8.3 percent DCS occurrence.

When the surface interval was increased to six hours, there was one neurological incident in six trials, for a DCS occurrence of 16.7 percent.

At nine hours, there was one neurological incident in seven trials, for a 14.3 percent DCS occurrence.

The DCS occurrence was higher at six and nine hours than at three hours because the six- and nine-hour intervals had to be rejected after a single case of neurological DCS. The three-hour surface interval, on the other hand, was not rejected until 36 trials were conducted. That may be due simply to luck, but this is a classic example of having a very low level of confidence in the data for the six- and nine-hour surface intervals because there were so few trials. It is reasonable to suppose that if we could conduct more trials at six and nine hours, the DCS percentage would be lower.

The correlation between the percentage of Ambiguous DCS and the surface interval was statistically significant, suggesting that the ambiguous symptoms were a mild form of DCS. At a three-hour surface interval, for example, there was a 10 percent estimated risk of Definite DCS and nearly a 20 percent estimated risk of Ambiguous DCS. The observed DCS percentage at six and nine hours does not follow the estimated trend due to the low number of studies that were conducted.

You could decide to wait for 12 hours before your flight, and have an estimated 1 percent risk of Definite DCS and approximately 2 percent risk of Ambiguous DCS. If you want to keep your level of risk at zero, then don't dive or don't fly. Every time you dive, you are subjecting yourself to a risk of DCS. No tables guarantee absolute safety.

In making decisions about risk, you can look to existing guidelines and practical experience for clues, such as the 12-hour flying-after-diving guideline. The estimated DCS probability for this surface interval is about 1 percent. Another clue is the estimated probability for a 55-minute dive to 60 feet. The estimated risk of DCS is 0.5-1.0 percent for this dive. We have far to go before we are comfortable with our estimates of DCS probability, but the information we've gathered thus far is already making decompression safety less mysterious.

If these decisions seem arbitrary to you, keep in mind that there is no "right" or "wrong" when deciding whether a profile is "safe" or "unsafe." Whatever preflight surface interval you choose, whether by guess or by probability estimate, there will be some DCS risk - there is no way to avoid it. Our goal at DAN is to develop information that can help you, the diver, make these choices as rationally as possible.

DON'T TRY THIS AT HOME!!!

Our conclusions shouldn't be tried "in the field" as gospel. All of our exposures at present are dry, resting dives under carefully monitored conditions. In no way should these results be construed as guidelines at this point. Wet, working dives may give very different results. Further testing is required before definitive guidelines can be developed.

Well, we haven't succinctly answered the question of "How long should I wait after diving before I fly?", but that wasn't the real purpose of this article. We wanted to make you think about what "safety" is and how it is determined. If we had all the answers, DAN Research could close up shop and take jobs teaching diving where the water is always clear and warm and the sun is always bright.

Instead, we will continue to focus our attention on these and other issues - and teach diving when and where we can. And we've moved into new levels of flying after diving research, studying flying after repetitive diving. We will relay our progress on this in later articles, as well as discuss how the number of tests we conduct affects the uncertainty or confidence in our probability estimates.